The crude oil from which gasoline and other liquid hydrocarbon fuels are derived generally comprises a diverse mixture of hydrocarbons and other compounds which boil over a wide range. Those components boiling at the lower end of this range (between about 100.degree. and 650.degree. F.) are in many cases recovered from the crude oil by atmospheric distillation. The higher molecular weight, high boiling components of crude oil, however, are not directly suitable for use in gasoline or other premium liquid hydrocarbon fuels.
In order to maximize the desired product yield from the crude, the petroleum refining industry has developed processes for cracking the higher molecular weight components into smaller molecules which boil over a lower temperature range. Among the most widely used of these methods is that known in the industry as fluid catalytic cracking (FCC). In the generalized FCC process, a vaporized hydrocarbon feedstock is contacted at an elevated temperature with a cracking catalyst. When the desired degree of cracking has been achieved, the vapor product is separated from the catalyst. The catalyst containing carbonaceous deposits is sent to a regenerator. In the regenerator the carbon is removed by burning in air and the catalyst activity restored. The catalyst is then generally recycled for the treatment of additional feedstock.
Crude oil usually contains a variety of components in varying amounts which reduce the efficiency of FCC processes. Among these are coke precursors (asphaltenes, polynuclear aromatics, etc.), heavy metals (nickel, iron, copper, vanadium, etc.) and lighter metals (sodium, potassium, etc.). The lighter metals can often be removed economically by conventional desalting operations forming a part of the standard pretreatment of crude oil prior to use in catalytic cracking or in the preparation of the heavier fuels; in some cases, however, caustic soda is used for corrosion control, which may lead to further sodium contamination. The coke precursors and heavy metals generally have been more troublesome.
The heavy ends of many crudes are particularly high in coke precursors and heavy metals which are undesirable in catalytic cracking feed stocks and in products such as heavy fuel, where ash specifications are sometimes important. The undesirable coke precursors and metal-bearing compounds present in the crude tend to become concentrated in the residues of atmospheric and vacuum distillations, commonly called atmospheric and vacuum residua or "resids", because most of them are of high boiling point. The present invention provides an economically attractive method for selectively removing and utilizing these undesirable components from whole crudes and from resids.
As used herein, the terms "residual stocks", "resids" and similar terminology include any petroleum fraction remaining after fractional distillation to remove some more volatile components. In that sense, "topped crude" remaining after distilling off gasoline and lighter fractions is a resid.
When catalytic cracking was first introduced in the petroleum industry during the 1930's, the process constituted a major advance over the previous techniques for increasing the yield of motor gasoline from petroleum. Today, the catalytic cracker is the dominant unit of a petroleum refinery. As installed capacity of catalytic cracking has increased, there has been increasing pressure to charge to these units greater proportions of the crude entering the refinery. Two very effective restraints have limited the extent to which this has been practicable, particularly in existing FCC's: the coke precursor content and the heavy metals content of the feed. As these values rise, the capacity and efficiency of the catalytic cracker are adversely affected.
Polynuclear aromatics, asphaltenes and other coke precursors tend to break down during the catalytic cracking process to form coke. This coke deposits on the active surface of the catalyst, thereby reducing its activity level. In general, the coke-forming tendency or coke precursor content of an oil can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolyzed. This value is accepted in the industry as a measure of the extent to which a given feedstock tends to form coke when treated in a catalytic cracker. One of the accepted methods for making this evaluation is the Conradson Carbon test. When a comparison of catalytic cracking feedstocks is made, a higher Conradson Carbon number (CC) reflects an increase in the portion of the charge converted to "coke" deposited on the catalyst.
It has been conventional to burn off the inactivating coke with air to "regenerate" the active surfaces, after which the catalyst is returned in cyclic fashion to the reaction stage for contact with and conversion of additional feedstock. The heat generated in the burning regeneration stage is recovered and used, at least in part, to supply heat for vaporization of the feedstock and for the cracking reaction.
The regeneration stage generally operates under a maximum temperature limitation in order to avoid heat damage to the catalyst. Since the rate of coke burning is a function of temperature, it follows that in any regeneration stage there is a limit to the amount of coke which can be burned in unit time. As CC of the charge stock is increased, coke-burning capacity becomes the limiting factor, often requiring a reduction in the rate of charge to the unit. Moreover, part of the charge has been diverted to an undesirable reaction product, reducing the efficiency of the process.
Metal-bearing fractions contain, inter alia, the heavy metals nickel and vanadium. When present in the charge, these metals are deposited almost quantitatively on the catalyst as the molecules in which they occur are cracked. The deposits of these metals build up over repeated cracking cycles to levels which become very troublesome. Some of these metals unfavorably alter the chemical composition of the catalyst. For example, vanadium tends to form fluxes with certain components of common FCC catalysts, lowering their melting point to the degree that sintering begins at FCC operating temperatures.
The heavy metals present in crude oils are also potent catalysts for the production of coke and hydrogen from the cracking feedstock. The lowest boiling fractions of the cracked product--butane and lighter--are processed through fractionation equipment to recover components of greater value than as fuel for the furnaces. This fraction comprises primarily propane, butane and olefins of like carbon number. Hydrogen, being incondensible in the "gas plant", occupies space as a gas in the compression and fractionation train. When an excessive amount is produced by high metal-content catalyst, this can easily overload the system, causing a reduction in the charge rate at which the catalytic cracking unit and auxiliaries remain operative.
In heavy fuels used in stationary furnaces, turbines and large stationary and marine diesel engines, quality is a significant factor. The overall quality of heavy fuels such as Bunker Oil and heavy gas oils is adversely affected as it becomes necessary to prepare these from crudes of high CC, metals and salt contents. An additional complication is that petroleum ash, particularly that containing vanadium and sodium, attacks furnace refractories and turbine blades.
These problems have long been recognized in the art and many expedients have been proposed. One approach uses thermal conversion producing large quantities of solid fuel as coke. This type of process has therefore been characterized as "coking". Two varieties are presently practiced commercially. One is known as delayed coking, in which the feed is heated in a furnace and passed to large drums maintained at 780.degree.-840.degree. F. During a long residence time at this temperature, the charge is converted to coke and distillate products are taken off the top of the drum for recovery of "coker gasoline", "coker gas oil" and gas. The other variety of coking process now in use employs a fluidized bed of coke in the form of small granules at about 900.degree. to 1050.degree. F. The resid charge undergoes conversion on the surface of the coke particles during a residence time on the order of two minutes, depositing additional coke on the surfaces of the particles in the fluidized bed. Coke particles are transferred to a bed fluidized by air to burn some of the coke at temperatures upwards of 1100.degree. F.; the thus-heated residual coke is then returned to the coking vessel for conversion of additional charge.
Coking does reduce metals and Conradson carbon in the production of a distillate from residuum. The distillate, however, is refractory for subsequent conversion or desulfurization processes. Moreover, coking produces a coke product high in sulfur and ash, and thus of poor quality. Typically, petroleum coke sells for 1/5 its heat value.
The known coking processes induce extensive thermal cracking of components which would be valuable as FCC charge, resulting in the production of gasoline of lower octane number than would be obtained by catalytic cracking. The gas oils produced are olefinic, containing significant amounts of diolefins which are prone to degradation to coke in furnace tubes and on cracking catalysts. It is, therefore, often desirable to treat these gas oils by expensive hydrogenation techniques before charging to catalytic cracking or blending with other fractions for fuels.
Catalytic charge stock and fuel stocks may also be prepared from resids by "deasphalting", in which an asphalt precipitant such as liquid propane is mixed with the oil. Metals and Conradson Carbon levels are significantly reduced, but a low yield of deasphalted oil is recovered.
Solvent extractions and various other techniques have also been proposed for preparation of FCC charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by chemical selection, rejecting from the charge stock aromatic compounds which can crack to yield high octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica gel has also been proposed (Shuman and Brace, Oil and Gas Journal, p. 113 (Apr. 6, 1953)).
In the 1950's, much work centered on the so-called Houdresid process for the conversion of crude oils into gasoline and other products. This type of process employed catalyst particles of a "granular" size substantially larger than the typical FCC catalyst particles in a compact gravitating bed. In spite of a low yield of product relative to fluid catalytic cracking of lighter gas oils, the Houdresid process offered the advantage of a decreased process sensitivity to high metals level. The heavy metals accumulating on the surfaces of the catalyst particles were apparently removed to some extent by an attrition process, whereby the outer layers of metal-contaminated catalyst were removed. Nonetheless, the Houdresid process is also unsatisfactory in terms of both economy and productivity.
U.S. Pat. Nos. 2,462,891 and 2,378,531 disclose processes utilizing a solid heat transfer medium to vaporize and preheat catalytic cracking charge stock. Heat from a catalytic regenerator is employed. The object of these processes is to vaporize the total quantity of a catalytic charge stock. It is, however, recognized that a heavy portion of the charge may remain in liquid state and be converted to vaporized products of cracking and coke by prolonged contact with the heat transfer material, a conversion related to the coking processes earlier noted. The use of solid heat transfer agents to induce extensive cracking of hydrocarbon charge stocks at the high temperatures and short reaction times which maximize ethylene and other olefins in the product has also been disclosed. An example of such teachings is U.S. Pat. No. 3,074,878.
U.S. Pat. No. 2,472,723 proposes the addition of an adsorptive clay to the charge for a catalytic cracking process. The clay is used on a "once-through" basis to adsorb the polynuclear aromatic compounds which are believed to be coke precursors and thereby reduce the quantity of coke deposited on the active cracking catalyst also present in the cracking zone.
U.S. Pat. No. 4,263,128 discloses a process for upgrading petroleum and residual fractions thereof, in which whole crude and bottoms fractions from distillation of petroleum are upgraded by high-temperature, short-time contact with a fluidizable solid of essentially inert character to deposit high boiling components of the charge on the solid. In this manner, Conradson Carbon values, salt content and metal content are reduced to levels tolerable in catalytic cracking. The upgraded hydrocarbon fraction may be supplied to a fractionator. The high temperature contactor thus serves as heater for the crude, in addition to improving the quality of the fractions derived by distillation. The disclosed process calls for the use of an inert solid of low surface area of a size of about 20 to 150 micron particle diameter, which is mixed with the resid or petroleum charge in a riser. The oil is introduced at a temperature below the thermal cracking temperature in admixture with steam and/or water to reduce the partial pressure of volatile components of the charge. The catalytically-inert solid is supplied to a rising column of charge at a temperature and in an amount such that the mixture is at a temperature of upwards of 700.degree. F. to 1050.degree. F., which is sufficient to vaporize most of the charge. The process is preferably conducted in a contactor very similar in construction and operation to the riser reactors employed in modern FCC units.
Co-pending U.S. patent application Ser. No. 299,361 discloses a selective vaporization process in which heavy charge stocks such as whole crudes, topped crudes, resids and the like are contacted with an inert, finely divided solid material in a confined vertical column under suitable conditions to deposit heavy components of high CC and/or metal content on the solids and vaporize other components of the charge. Various hydrocarbons are separated at the top of the column from inert solids bearing the unvaporized components as a deposit thereon. The vapors are promptly cooled to a temperature below that at which substantial thermal cracking occurs and are processed as desired in a catalytic cracker or the like. According to some embodiments of the process, contact is effected in a riser. In other embodiments, a rising column of inert solids in steam, hydrocarbon gases or mixtures of the two is established and the direction of flow is subsequently reversed to a confined descending column into which the charge is injected.
Although both U.S. Pat. No. 4,263,128 and U.S. patent application Ser. No. 299,361 disclose processes which provide results superior to the prior art methods for upgrading petroleum or residual fractions thereof, the industry for obvious reasons is constantly searching for methods which maximize the yield of high-hydrogen petroleum components and minimize coke deposits. In particular, minimization of the contact time between the petroleum charge and the contact material to that period which would allow for essentially no cracking of high-hydrogen components is a major goal of selective vaporization processes. In addition, the best possible stripping and rapid disengagement of the petroleum charge from the contact material would maximize liquid yield and facilitate control of burner temperatures below their metallurgical limit. A minimization of contact material abrasion and plant erosion due to contact material circulation is also desirable.
In spite of the improvements achieved through the selective vaporization processes described above, it has been very difficult in practice to get the absolute minimum contact times and the desired intimate mixing in some existing riser contactor units. This is because of mechanical limitations of some typical contactor units, which in general comprise a vertical conduit enclosing the hydrocarbons, diluents and fluidizable contact material. First, the correct hydraulics are necessary to ensure proper circulation. After adjustment of the burner and contactor pressures, however, a vertical conduit contact of such great length may be required that one often needs multiple injection and gas recycle systems to achieve the desired minimum contact times. The use of a hydrocarbon gas recycle obviously places an additional power load on the system. Multiple injection systems do result in lower contact times, but increase utility requirements. In a vertical upflow conduit contactor, there is also generally some slippage of the contact materials, increasing the contact time. Moreover, it may be difficult to get the desired intimate mixing in some systems. The hydrocarbon feedstock is normally injected vertically into the center of the conduit with the contact material on the periphery, or the feedstock is injected around the periphery of the conduit with the contact material in the center. Neither of these commonly employed methods necessarily provides optimum mixing.
In conclusion, an ideal system for upgrading petroleum feed stocks would achieve the following goals: (1) an immediate vaporization of the high hydrogen, low boiling components; (2) an optimum reaction time on the surface of the contact material for the heavier hydrocarbon components and metal bearing compounds; (3) a retention of the metals by the contact material, with a minimization of "poisoning"; (4) an optimum degree of "cracking" of the higher hydrocarbon components with a minimization or elimination of cracking of the lighter hydrocarbons; and (5) a rapid condensation of the uncracked hydrocarbon vapors free of metals and carbonaceous materials.